FIGS. 1A, 1B, and 1C depict conventional magnetic elements 10, 10′, and 10″ that can be used in magnetic memories. Note that recent reviews of developments in the field of magnetic memories can be found for example in: “Memories of Tomorrow”, by William Reohr et al., IEEE Circuits and Devices Magazine, pp. 17–27, September 2002; and “Magnetoresistive Random Access Memory Using Magnetic Tunnel Junctions”, by Saied Tehrani et al., Proceedings of the IEEE, vol. 91, no. 5, pp. 703–714, May 2003. The conventional magnetic element 10 is a spin valve and includes a conventional antiferromagnetic (AFM) layer 12, a conventional pinned layer 14, a conventional spacer layer 16 that is conductive and a conventional free layer 18. Other layers (not shown), such as seed or capping layer may also be used. The conventional pinned layer 14 and the conventional free layer 18 are ferromagnetic. Thus, the conventional free layer 18 is depicted as having a changeable magnetization 19. The conventional spacer layer 16 is nonmagnetic. The AFM layer 12 is used to fix, or pin, the magnetization of the pinned layer 14 in a particular direction. The magnetization of the free layer 18 is free to rotate, typically in response to an external magnetic field. Also depicted are top contact 20 and bottom contact 22 that can be used to drive current through the conventional magnetic element 10.
The conventional magnetic element 10′ depicted in FIG. 1B is a spin tunneling junction. Portions of the conventional spin tunneling junction 10′ are analogous to the conventional spin valve 10. Thus, the conventional magnetic element 10′ includes an AFM layer 12′, a conventional pinned layer 14′, a conventional space layer that is an insulating barrier layer 16′ and a conventional free layer 18′ having a changeable magnetization 19′. The conventional barrier layer 16′ is thin enough for electrons to tunnel through in a conventional spin tunneling junction 10′.
The conventional magnetic element 10″ includes an AFM layer 12′, a conventional pinned layer 14″, a conventional spacer layer that is a current confined layer 16″ and a conventional free layer 18″ having a changeable magnetization 19″. The conventional current confined layer 16″ is an inhomogeneous layer mixing areas of metallic conduction (hereafter termed conductive channels 15), with high resistivity regions (hereafter termed an insulating matrix 17) that may be insulators. The conduction between the ferromagnetic layers 14″ and 18″ is essentially confined to the conductive channels 15. The conventional magnetic element 10″ is thus termed a current confined magnetoresistance effect thin film structure. The conventional magnetic element 10″ is more fully described in the context of magnetoresistive hard disk drive read-out heads in: “The Applicability of CPP-GMR Heads for Magnetic Recording”, by M. Takagishi et al., IEEE Trans. Magn. 38, 2277 (2002).
Depending upon the orientations of the magnetization 19/19′/19″ of the conventional free layer 18/18′/18″ and the conventional pinned layer 14/14′/14″, respectively, the resistance of the conventional magnetic element 10/10′/10″, respectively, changes. When the magnetization 19/19′/19″ of the conventional free layer 18/18′/18″ is parallel to the magnetization of the conventional pinned layer 14/14′/14″, the resistance of the conventional magnetic element 10/10′/10″ is low. When the magnetization 19/19′/19″ of the conventional free layer 18/18′/18″ is antiparallel to the magnetization of the conventional pinned layer 14/14′/14″, the resistance of the conventional magnetic element 10/10′/10″ is high. To sense the resistance of the conventional magnetic element 10/10′/10″, current is driven through the conventional magnetic element 10/10′/10″. Typically in memory applications, current is driven in a CPP (current perpendicular to the plane) configuration, perpendicular to the layers of conventional magnetic element 10/10′/10″ (up or down, in the z-direction as seen in FIG. 1A, 1B, or 1C). In this configuration, current is driven between the top contact 20, 20′, 20″ and the bottom contact 22, 22′, and 22″, respectively.
In order to overcome certain issues associated with magnetic memories having a higher density of memory cells, spin transfer may be utilized to switch the magnetizations 19/19′/19″ of the conventional free layers 10/10′/10″. Spin transfer is described in the context of the conventional magnetic element 10′, but is equally applicable to the conventional magnetic elements 10 and 10″. Current knowledge of spin transfer is described in detail in the following publications: J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1 (1996); L. Berger, “Emission of Spin Waves by a Magnetic Multilayer Traversed by a Current,” Phys. Rev. B, vol. 54, p. 9353 (1996), F. J. Albert, J. A. Katine and R. A. Buhrman, “Spin-polarized Current Switching of a Co Thin Film Nanomagnet,” Appl. Phys. Lett., vol. 77, No. 23, p. 3809 (2000), “Conductance and exchange coupling of two ferromagnets separated by a tunneling barrier”, by J. Slonczewski, Phys. Rev. B 39, 6995 (1989), and “Observation of spin-transfer switching in deep submicron-sized an low-resistance magnetic tunnel junctions” by Y. Huai et al., Appl. Phys. Lett. 84, 3118 (2004). Thus, the following description of the spin transfer phenomenon is based upon current knowledge and is not intended to limit the scope of the invention.
When a spin-polarized current traverses a magnetic multilayer such as the spin tunneling junction 10′ in a CPP configuration, a portion of the spin angular momentum of electrons incident on a ferromagnetic layer may be transferred to the ferromagnetic layer. In particular, electrons incident on the conventional free layer 18′ may transfer a portion of their spin angular momentum to the conventional free layer 18′. This transfer of angular momentum can be considered a spin transfer torque acting on the free layer magnetization 19′. As a result, a spin-polarized current can switch the magnetization 19′ direction of the conventional free layer 18′ if the current density is sufficiently high (approximately 107–108 A/cm2) and the lateral dimensions of the spin tunneling junction are small (approximately less than two hundred nanometers). The threshold current at which spin transfer induced switching can occur is termed the critical current, Ic. In addition, for spin transfer to be able to switch the magnetization 19′ direction of the conventional free layer 18′, it is generally believed that the conventional free layer 18′ should be sufficiently thin, for instance, preferably less than approximately ten nanometers for Co. Spin transfer based switching of magnetization dominates over other switching mechanisms and becomes observable when the lateral dimensions of the conventional magnetic element 10′ are small, in the range of few hundred nanometers. Consequently, spin transfer is suitable for higher density magnetic memories having smaller magnetic elements 10′.
The phenomenon of spin transfer can be used in the CPP configuration as an alternative to or in addition to using an external switching field to switch the direction of magnetization of the conventional free layer 18/18′/18″ of the conventional magnetic element 10/10′/10″. For example, in the conventional magnetic element 10′, the magnetization 19′ of the conventional free layer 18′ can be switched from antiparallel to the magnetization of the conventional pinned layer 14′ to parallel to the magnetization of the conventional pinned layer 14′. Current is driven from the conventional free layer 18′ to the conventional pinned layer 14′ (conduction electrons traveling from the conventional pinned layer 14′ to the conventional free layer 18′). Alternatively, the magnetization of the free layer 18′ can be switched from a direction parallel to the magnetization of the conventional pinned layer 14′ to antiparallel to the magnetization of the conventional pinned layer 14′ when current is driven from the conventional pinned layer 14′ to the conventional free layer 18′ (conduction electrons traveling in the opposite direction.
The magnitude of the critical current, Ic, can be determined using the prevalent spin transfer spin-torque model described in J. C. Slonczewski, “Current-driven Excitation of Magnetic Multilayers,” Journal of Magnetism and Magnetic Materials, vol. 159, p. L1–L5 (1996), and further expanded in particular in: “Field dependence of magnetization reversal by spin transfer”, by J. Grollier et al., Phys. Rev. B 67, 174402 (2003). According to Slonczewski's model, the switching current density Ic for the free layer of a spin transfer stack is proportional to:αtMs[Heff−2πMs]/g(θ)                where:        α=the phenomenological Gilbert damping parameter;        t=the thickness of the free layer;        Ms=saturation magnetization of the free layer;        Heff=effective field for the free layer;        g(θ) reflects the spin-transfer efficiencyThe effective field, Heff, includes the external magnetic field, shape anisotropy fields, in-plane and out-of-plane (i.e. perpendicular) anisotropies, and dipolar and exchange fields. The perpendicular anisotropy typically arises from crystalline anisotropy. The term g(θ) depends on the relative angular orientations of the magnetizations of the conventional pinned layer 14′ and the conventional free layer 18′.        
Thus, the critical current Ic is proportional to the Gilbert damping parameter a of the conventional free layer 18′. This is believed to be equally applicable to spin transfer in conventional spin valve magnetoresistance effect element such as 10 and conventional current confined magnetoresistance effect element 10″. The Gilbert damping parameter α is a dimensionless parameter, which quantifies the level of dynamic damping experienced by the conventional free layer magnetization 18′. Assuming the remaining factors remain the same, a reduction in a results in a proportional reduction of Ic, while an increase in a results in a proportional increase of Ic. For a thin conventional magnetic free layer 18′ embedded in a multilayer structure, it has been shown that the total damping coefficient, α, can be in general broken into three contributions:α=α0+(δαout+δαin)t0/tf                where:        α0=the intrinsic damping parameter;        δαout=surface contribution originating from processes taking place at the outer interface of the free layer, for example between the conventional free layer 18′ and the top contact 20′;        δαin=surface contribution originating from processes taking place at the inner interface of the free layer, for example between the conventional free layer 18′ and the barrier layer 16′;        t0=arbitrary scaling length;        tf=thickness of the free layer expressed in nanometersThe intrinsic damping parameter α0 is dependent only on the material used to create the conventional free layer 18′. The arbitrary scaling length, t0, is conveniently taken equals to three nanometers without loss of generality. The thickness of the conventional free layer 18′, tF, is the thickness of the free layer expressed in nanometers.        
The inner surface contribution to the damping parameter, δαin, depends on the detail of the structure and composition of the interface between the conventional free layer 18′ and conventional barrier layer 16′, the conventional barrier layer 16′ itself, possibly the interface between the conventional barrier layer 16′ and the conventional pinned layer 14′, and the conventional pinned layer 14′. In particular, the magnetic element 10′ may experience a significant and detrimental contribution of δαout that can be traced back to “spin pumping” taking place at the top (outer) interface of the free layer 10′. Spin pumping damping is generated by losses of angular momentum from the time dependent magnetization of the conventional free layer 18′ by exchange coupling with the free electrons able to leave the free layer into the top contact 20′. Such effects are described in details for example in: “Dynamic stiffness of spin valves” Y. Tserkovnyak et al., Phys. Rev. B 67, 140404 (R) (2003). Such spin pumping induced damping is a limiting factor in the ability to decrease Ic to desirable levels for magnetoresistance effect thin film structures as known in the prior art, with free layer thickness typically ranging from one to five nanometers.
Thus, although spin transfer functions as a mechanism for switching the conventional magnetic elements 10/10′/10″, one of ordinary skill in the art will readily recognize that a high current density is typically required to induce switching for the conventional magnetic elements 10/10′/10″. In particular, the switching current density is on the order of a few 107 A/cm2 or greater. Thus, a high write current is used to obtain the high switching current density. The high operating current leads to design problems for high density MRAM, such as heating, high power consumption, large transistor size, as well as other issues.
One of ordinary skill in the art will readily recognize that one mechanism for reducing the switching current is to employ a dual magnetoresistive element. FIG. 2 depicts a conventional dual magnetoresistance element 50. The dual magnetoresistance element 50 includes an AFM layer 52, a first conventional pinned layer 54, a first conventional spacer layer 56, a conventional free layer 58, a second conventional spacer layer 60, a second conventional pinned layer 62, and a second AFM layer 64. Other layers (not shown), such as seed or capping layer may also be used. A bottom contact 66 and a top contact 68 are also shown. The conventional pinned layers 54 and 62 and the conventional free layer 58 are ferromagnetic. Thus, the conventional free layer
58 is depicted as having a changeable magnetization 59. The conventional spacer layers 56 and 60 are nonmagnetic. The conventional spacer layer 56 and 60 could be conductive, tunneling barrier layers, or current confined layers. Thus, the conventional spacer layers 56 and 60 correspond to the layers 16, 16′, and/or 16″ depicted in FIGS. 1A–1C. Referring back to FIG. 2, the AFM layers 52 and 64 are used to fix, or pin, the magnetization of the pinned layers 54 and 62, respectively, in a particular direction. The magnetization of the free layer 58 is free to rotate, typically in response to an external magnetic field.
When a spin-polarized current traverses the conventional magnetic element 50, the magnetization 59 of the free layer 58 can be switched using spin transfer. Angular momentum is carried by the electrical current between the first conventional pinned layer 54 and the conventional free layer 58. This current generates a first spin transfer torque, T1, that acts on the conventional free layer 58. Angular momentum is also carried by the electric current between the second conventional pinned layer 62 and the conventional free layer 58. This current generates a second spin transfer torque, T2, that also acts on the conventional free layer 58. With proper choices of the layer thicknesses, composition, and magnetization orientation of the pinned layers 54 and 62, it is possible to create a conventional magnetic element 50 in which the two torques T1 and T2 add in the desired fashion. As a result, a significant reduction in the critical current Ic might be obtained.
Although the critical current, Ic, can be reduced, one of ordinary skill in the art will readily recognize that such conventional magnetic elements 50 can suffer from drawbacks that adversely affect the performances of the conventional magnetic element 50 and magnetic memory that can eventually be built out of it.
First, presume that the second conventional spacer layer 60 is a conductor. As discussed above, the critical current Ic is proportional to the Gilbert damping parameter α of the conventional free layer 58. As indicated above, α can be broken into α0, δαout, and δαin. A conventional magnetic element 50 utilizing a second conventional spacer layer 60 that is metallic experiences a significant and detrimental additional contribution from δαout that can be traced back to “spin pumping” taking place at the top interface of the free layer 58 in conjunction with the second magnetic pinned layer 62. Such “spin pumping” is analogous to the spin pumping described above.
If the second conventional spacer layer 60 is a tunnel barrier, “spin pumping” at the interface between the free layer 58 and the second non conventional spacer layer 60 does not adversely affect the critical current. However, one of ordinary skill in the art will readily recognize that in order for the sub-structure composed of the free layer 58, the first conventional spacer layer 56, and the first conventional pinned layer 54 to maximize the magnetoresistance effect of the conventional magnetic element 50, the first conventional spacer layer 56 is also a tunnel barrier and that the areal resistance of the second spacer layer 60 is significantly smaller than the areal resistance of the first tunnel barrier 56. In such a conventional dual spin tunneling junction 50, the areal resistance of the second conventional spacer layer 60 is typically ten times smaller than the areal resistance of the first conventional spacer layer 56. Furthermore, for the conventional magnetic element 50 to be capable of being written to using spin transfer, even the first conventional spacer layer 56 has a relatively small areal resistance, typically below twenty Ω.μm2. To manufacture such a conventional magnetic element 50 including two tunneling barriers 56 and 60 with such low areal resistances is very difficult. Consequently, would be highly desirable to identify an alternate design for dual magnetoresistance effect thin film structures, allowing to suppress the spin pumping contribution to δαout without requiring the stacking of two low areal resistance magnetic tunnel junctions.
Finally, if the second conventional spacer layer 60 is a current confined layer and if the areal resistance of the interface between the conventional free layer 58 and the second conventional spacer layer 60 is larger than 0.1 Ω.μm2, then it can be shown that the spin pumping contribution to the dynamic damping for the conventional free layer 58 is effectively suppressed. However, for conventional values of the various material and geometrical parameters defining such a conventional magnetic element 50, it can be determined using the magneto-electronic circuit theory that the overall spin transfer torque created on the conventional free layer 58 for a given current I flowing between the two contact electrodes 66 and 68, is only larger by a factor of three to four by comparison with a simple magnetoresistance effect film structure as depicted on FIGS. 1A–1C. One can find a detailed description of the magneto-electronic circuit theory, and some of it's applications to spin transfer in the following publication: A. Brataas, Y. V. Nazarov, and G. E. W. Bauer, “Finite-Element Theory of Transport in Ferromagnet—Normal Metal Systems”, in Phys. Rev. Lett. 84, 2481 (2000). It is thus desirable to create improved dual magnetoresistance effect thin film structures that exhibits even larger spin transfer torque on the conventional free layer 58 for the same current.
Accordingly, what is needed is a system and method for providing a dual magnetoresistance element that can be switched using spin transfer at a lower current and that consumes less power. The present invention addresses such a need.